TRICKY GOING

The feat of exploring Hang Son Doong begins right at the entrance, where author Mark Jenkins picks his way across mossslick boulders. A local hunter years earlier had discovered the opening after seeing trees shake from underground wind gusts. In 2009, a British-led expedition began surveying "mountain river cave," returning last year to travel even deeper inside.

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GETTING WET

Expedition members ford knee-deep Rao Thuong River as it tunnels through limestone. The river eventually drains beneath the main level, carving new passages more than a thousand feet below. The year before, with the water dangerously high and fast, the cavers had to rope themselves across.

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CAVERNOUS

A towering stalagmite dwarfs a climber surveying a route through the central passage. A half mile farther on, an expedition member is barely visible atop a formation called Hand of Dog, a landmark for the explorers.

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SKYSCRAPER LARGE

The powerful beam of a headlamp helps illuminate the huge dimensions of Han Son Doong's main passage, considered the world's largest underground space. As measured by laser, the entire passage extends for 2.5 miles and reaches higher than 600 feet. This stretch, lit also by the photographer's lights, could accommodate a half-mile block of 40-story buildings.

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INCOMING

"It sounded like a roaring train," said expedition member Gareth "Sweeny" Sewell, describing the split second before a waterfall burst through Watch Out for Dinosaurs, a sinkhole opening. A rare dry-season downpour unleashed the cascade.

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MAGIC WORLD

Daylight streaming through Watch Out for Dinosaurs turns a rock formation into a fairytale castle perched on a hill. It even has a moat—a reflecting pool filled by a passing storm.

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CIRCUITOUS ROUTE

Expedition leaders Deb and Howard Limbert show the way through an algae-skinned maze. The intricate cavescape was formed from deposits precipitated by calcite-rich waters as they overflowed a series of shallow pools. Since 1990 the Limberts have discovered and explored dozens of caves in the remote Vietnam-Laos border region.

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IT'S A JUNGLE DOWN THERE

From above, climber Sweeny Sewell sees a lush, bewitching forest. At ground level, however, orange-suited hikers struggle through a tangled obstacle course in the wryly named Garden of Edam. Long ago the ceiling collapsed into the cave. Fed by light and rain, plants colonized the rubble. Creatures have moved in as well, including snakes, birds, and monkeys.

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UNDERGROUND VISION

Used to damp, cramped, pitch-black crawl spaces back home, British cavers gloried in the airy, light-filled vista of the Garden of Edam. The expedition pitched camp at the edge of the skylight in the driest section of the cave.

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CAVE PEARLS

Expedition geologist Darryl Granger of Purdue University marvels at an unusually rich concentration of spherical rocks called cave pearls. They formed drip by drip over the centuries as calcite crystals layered themselves around a grain of sand. Slowly enlarging, some cave pearls can grow as big as grapefruits.

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RARE COLLECTION

Cave pearls, ranging in size from marbles to baseballs, fill a section of dried-out pools. The trove likely will stay intact, since the Vietnamese government has indicated that it will keep the cave off-limits to tourists visiting Phong Nha-Ke Bang National Park.

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BLOCKING THE WAY

A 200-foot cliff of flowstone and mud, dubbed the Great Wall of Vietnam, thwarted the advance of the first expedition. The cavers returned to see what was on the other side—the eternal urge of explorers.

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UP AND OVER

Scaling the Great Wall called for two days of risky advance work by Sweeny Sewell to drill climbing bolts. Here he doublechecks his efforts so the team can follow. He works quickly; in the coming wet season, water can rise as high as the white streak in the rock. And what did the explorers find on the other side? A little more cave, an exit, and immense satisfaction.